Telecommunication systems of today, such as, for example Global System for Mobile communications (GSM), Wideband Code Division Multiple Access (WCDMA) systems, (Long Term Evolution) LTE and Universal Mobile Telecommunications System (UMTS), include at least one User Equipment (UE) or Mobile Station (MS), a Radio Access Network (RAN) and a Core Network (CN). (Hereafter in this document, UE will be used as a generic term for UE, MS and any other mobile transceiver in such a system.) The UE attaches to the CN through the RAN in order to get service connectivity, thereby establishing a UE context in the CN and RAN. The UE context defines values for a number of various network parameters for the created session, the network here including RAN and CN.
Once the UE context has been established between the UE and the network, a communication link between the UE and the network may be formed to transmit modulated information carrying signals based on those network parameters. When information carrying signals are no longer transmitted on the communication link, the UE does not necessarily have to maintain this communication link. Instead, after such a communication has terminated, the UE is usually set to idle mode to save battery power. In the UE idle mode, the UE context in RAN is typically removed but the UE context in CN still remains.
One of the most important reasons for maintaining UE context in CN for UE idle mode is to support Mobility Management (MM). The MM function, for a UE which is not actively engaged in a communication, keeps track of the location of the UE with the accuracy of a defined area in the system.
Such defined areas have been denoted differently in different systems. For instance, this area is denoted as a Tracking Area (TA) in SAE/LTE (System Architecture Evolution/Long Term Evolution), Location Area (LA) or Routing Area (RA) in GSM, and Universal Terrestrial Radio Access Network (UTRAN) Registration Area (URA), LA of RA in UMTS. Hereafter in this document, unless otherwise stated, TA will be used as a generic term for TA, LA, RA, URA, and other corresponding denotations, which is in correspondence with what is used in the 3GPP standardization work. (See chapter 7.3.1 in “3GPP TR 23.882 V1.9.0 (2007-03)”.)
In more general terms, a telecommunication system has a coverage area being made up of sub-areas in which a UE can be registered in when being in idle mode. The system will in UE idle mode be able to locate the UE to such a sub-area. Such a sub-area can thus, for example, consist of the above defined TAs, but also of any other area, in which a UE is registered when being in UE idle mode for the purpose of allowing the system to keep track of the UE. Sub-areas in a system can be arranged in one or more layers. For cellular systems, these sub-areas include at least one cell, and normally they include a plurality of cells.
Hereafter, for simplicity reasons, the present invention will be explained mainly in the terms of TAs and LTE. The invention is, however, not limited to this specific implementation. The present invention is instead applicable to all communication systems having sub-areas as defined above.
In each cell of the system, the corresponding TA Identifier (TA ID) is broadcasted on the air interface, so that a UE roaming these cells can know whether its TA has changed or not. The UE does this by comparing its last registered TA ID with the one received from the air interface in the current cell. In the case where the UE enters a new TA, a particular MM procedure, Tracking Area Update (TAU), is carried out to register the presence of the UE in the new TA. (TAU is here used as a generic term for the corresponding procedures in LTE, GSM, UTRAN and UMTS. Thus, the term TAU here also includes, e.g., Location Area Update (LAU) and Routing Area Update (RAU).) When the network gets incoming data for a UE in idle mode, the network locates the UE to the TA in which the UE is registered, and then pages the UE on a paging channel in the cells belonging to that TA.
TA configuration needs to take both LA/RA/TA update and paging load into account. TA should be configured as small as possible to keep the paging load within an acceptable level, but as large as possible to avoid too much TAU signaling.
In LTE, Multiple Tracking Area (MTA) has been proposed to enable Non Access Stratum (NAS) signaling optimization by dynamic TA configuration. In MTA, the UE is registered in one or more TAs, the one or more TAs constituting a registration area, in which the UE is registered during idle mode. Conceptually, the use of such a registration area is quite similar to existing RA in GSM EDGE Radio Access Network (GERAN)/UTRAN today. Each cell in the network broadcasts one TA ID for each CN operator. When the idle UE enters a TA in which it has never been registered, the UE will initiate a TAU procedure to register in this TA. Registration area registration allows for more than one TA to be assigned to a UE through a single TAU procedure, and the additional TAs are treated in the same manner as in the single TA case. As long as the UE roams within TAs it has already been assigned to, it does not need to perform any other TAU than periodical updates.
FIG. 1 shows an example of such a registration area. Here, a UE being located in the border area between three TAs registers with all of the three tracking areas, TA1, TA2 and TA3. Within the area being defined by the outermost line, enclosing an area consisting of TA1, TA2 and TA3, TAU signaling is reduced considerably. TAU signaling is here reduced because only periodic updates need to be performed, since no TAUs have to be performed when roaming from one TA to another within this area.
Employing the registration area concept, it is possible to avoid unnecessary TAU and paging overhead by dynamically reconfiguring TAs assigned to a particular UE according to a certain criteria. A number of ideas on how to dynamically reconfigure TAs for a UE has been discussed within the 3rd Generation Partnership Project (3GPP), for example, has UE-reporting based network-assigned TA approach been proposed, where UE provides some information assisting CN to detect a stationary UE and accordingly assign a small number of TAs to it, which helps reduce paging signaling efficiently.
In a typical registration area concept, which has been specified in the LTE standardization work of today, only one CN identity, called SAE Temporary Mobile Subscriber Identity (S-TMSI), is used for the UE in all TAs being assigned to the UE, i.e., the TAs present in the assigned TA list. When the UE moves to a new cell belonging to a new TA but still in the assigned TA list, it can use one of the old TA IDs plus the S-TMSI as a valid identity and does not need to perform a TAU.
The TAs assigned to the UE may either be managed by one single Mobile Management Entity (MME) node of the CN or more than one MME nodes. In the case of more than one MME nodes, each of the MME nodes maintains a MM context for the UE and they coordinate with each other.
In the case where the UE has registered with multiple Radio Access Technologies (RATs), i.e., the TAs in which the UE is registered utilize different RATs, the system typically maintains multiple mobile management contexts, each of which assign one identifier for the UE. For example, one S-TMSI is assigned in LTE and one Packet-TMSI (P-TMSI)/TMSI is assigned in UMTS.
In prior art systems, where UEs in idle mode are registered in multiple sub-areas, such as TAs, there exists a problem related to determination of network parameters in the system, such as, for example, paging related network parameters.
In order to be able to determine these network parameters in a correct way, it is necessary for the system to know the behavior of the UE in the system. In the prior art systems, it has not been possible for the systems to closely enough follow the behavior of the UE, which has resulted in an inaccurate determination of the network parameters in the system.
Inaccurate determination of the network parameters has further resulted in a sub-optimized overall performance of the prior art systems.